Languages can be a bit…too willing to share. Pretty much every natural language in existence has borrowed something from its neighbors. Some (like English) have gone farther than others (like Icelandic), but you can’t find a single example out there that doesn’t have some borrowing somewhere.
For the conlang creator, this presents a problem. Conlangs, by definition, have no natural neighbors. They have no history. They’re, well, constructed. This means they can’t undergo the same processes of borrowing that a natural language does. For some (particularly auxlangs), that’s a feature, not a bug. But those of us making naturalistic conlangs often want to simulate borrowing. To do that, we have to understand what it is, why it happens, and what it can do for us.
On loan
Most commonly, borrowing is in the form of loanwords, which are exactly what they sound like. Languages can borrow words for all sorts of reasons, and they can then proceed to do terrible things to them. Witness the large number of French loans in English, and the horrified shudders of French speakers when we pronounce them in our Anglicized fashion. Look at how terms from more exotic languages come into English, from chop suey to squaw to Iraqi. Nothing is really safe.
Pronunciations change, because the “borrowing” language might not have the same sounds or allow the same syllables. Meanings can subtly shift in a new direction, as cultural forces act on the word. Grammar puts its own constraints on loanwords, too; languages with case and gender will have to fit new words into these categories, while those without might borrow without understanding those distinctions.
But let’s take a step back and ask ourselves why words get borrowed in the first place. There are a few obvious cases. One, if the borrowing language doesn’t already have a word representing a concept, but a neighbor does, then it doesn’t take a psychic to see what’s going to happen. That’s how a lot of agricultural and zoological terms came about, especially for plants and animals of the Americas and Australia. It’s also how many of Arabic scientific words came into English, such as alcohol and algebra.
Another way loanwords can come about is through sheer force. The classic example is the Norman Conquest, when Anglo-Saxon fell from grace, replaced in prestigious circles by Norman French. Another “conquest” case is Quechua, in the Andes, where Spanish took the place of much of the native vocabulary. And then there’s Japanese, which borrowed a whole system of writing from China, complete with instructions on how to read it; just about every Chinese character got reinterpreted in Japanese, but their original—yet horribly mangled—Chinese pronunciations stuck around.
Third, a relative difference in status, where a foreign language is seen as more “learned” than one’s own, can drive borrowing. That’s one reason why we have so many Latin and Greek loans in English, especially “higher” English. Educated speakers of centuries past looked to those languages for guidance. When they couldn’t find the right word in their native tongue, the first place they’d look was the classics.
Taking more
Words are the most commonly borrowed item in language, but they’re not the only thing that can be taken, and they’re not always taken in isolation. English pronouns, for example, are a curious mix of native terms passed down with only minor changes all the way from Proto-Germanic and beyond—I and me aren’t that much different from their equivalents in most other European languages. But in the third person, the he, she, they, and it, things get weird. Specifically, the plural pronouns they, them, and their are, in fact, borrowed. Imposed, if you prefer, as they seem to be a result of the Viking invasions of England in the tenth and eleventh centuries.
Other bits of grammar can be lifted, but the more complicated they are, the less likely it’s going to happen. There aren’t a lot of examples of languages borrowing case systems. (Getting rid of one already present, however, is a plausible development for a language suddenly spoken by a large number of foreigners, but that’s a different post.) Borrowing of pronoun systems is attested. So is heavy borrowing of numeral words; this one is particularly common among indigenous languages that never needed words for “thousand” and “million” before Westerners arrived on the scene.
As I said above, Japanese went so far as to import a script. So did Korean, Vietnamese, and quite a few other languages in the region. They all took from the same source, Chinese, because of the much higher status they perceived it to have. Others around there instead borrowed from Sanskrit. On our side of the world, you have things like the Cherokee syllabary, although it’s not a “proper” borrowing, as the meanings of symbols weren’t preserved.
One other thing that can be taken isn’t so much a part of grammar as it is a way of thinking about it. As part of its mass importation of Latin and Greek, English picked up the Latin style of word formation. Instead of full compounds, which English had inherited from its German forebears, Latin used a more purely agglutinative style, full of prefixes and suffixes that added shades of meaning. It’s from that borrowing that we get con- and pro-, sub- and super-, ex-, de-, and so many more.
Word of warning
It’s easy to go too far, though—some would say English did long ago. So where do we draw the line? That’s hard to say. For some conlangs, borrowings, if they’re used at all, might need to be restricted to the upper echelons of the vocabulary. The technical, scientific terminology common to the whole world can be used without repercussion. Nobody will call out a conlang set in today’s world for borrowing meter and internet and gigabyte. Similarly, place names are fair game. Beyond that, it’s a matter of style and personal preference. If your conlang really needs a lot of loans, go for it.
There’s one more thing to think about. Borrowings get “nativized” over time, to the point where we no longer consider words like whiskey or raccoon to be loans. It’s only those that are relatively new (karaoke) or visibly foreign (rendezvous) that we take to be imports. Even those quixotic attempts to purge the language of its outside influences miss quite a lot here; you wouldn’t find even the hardiest Anglo-Saxon revivalist wanting to change Shakespeare’s Much Ado About Nothing, but ado is a pre-Norman loanword.
So this gives you an out: some words can be loans, but they were borrowed so long ago that the speakers have all but forgotten where they came from. Any conlang set in Europe, for instance, wouldn’t be wrong in having a lot of Roman-era Latin loans. Asian conlangs would almost be expected to have an ancient crust of Chinese or Sanskrit, or a newer veneer of Arabic.
Whatever you do, it’s an artistic choice. But it’s a choice that can have a profound effect on your conlang’s feel. A few well-placed borrowings give a conlang a sense of belonging to the real world. And if you’re making your own world, then you can create your own networks of linguistic borrowing, based on that world’s history. The principles are the same, even if the names are changed.